Radio frequency (RF) power amplifiers are often used in portable battery operated wireless devices, such as cellular telephones. Extending the battery life is a key concern for users and manufacturers of these battery operated wireless devices. One of the key factors in determining the battery life of the battery operated wireless device is the power consumption of the RF power amplifiers. The RF power amplifiers are designed to operate into an optimal load impedance and are typically coupled to an antenna of the battery operated wireless device.
However, under a load mismatch condition, such as, for example, when the antenna of the battery operated wireless device approaches objects (e.g. metal structures, human contact, or the like), the load impedance of the RF power amplifier changes and the RF power amplifier draws excess current. In some cases, the current can exceed more than two times the current drawn under an optimal load impedance. When the RF power amplifier draws excess current, the battery life of the battery operated wireless device is reduced. In addition, the adjacent channel power ratio (ACPR) and error vector magnitude (EVM) linearity and distortion limits are often exceeded when the RF power amplifier draws excess current. This reduction in battery life and distortion limits of the battery operated wireless device is undesirable.
FIG. 1 shows a schematic diagram of a current sense circuit 100 according to the prior art. Current sense circuit 100 includes a sense circuit 102, a battery voltage Vbatt, an RFin signal, a RFout signal and a Vsense signal. Sense circuit 102 includes an output transistor Q1, an inductor L1, a resistor R1, and a comparator 104. As the current in output transistor Q1 increases, due to, for example, a mismatch condition, comparator 104 senses the voltage drop across resistor R1 and provides a corresponding voltage signal, i.e., Vsense signal.
However, the use of the series resistor in the battery path is disadvantageous because even under normal operation, the series resistor is always in series with the battery. Among other things, this reduces the efficiency of the output transistor Q1 and decreases the battery life of the battery operated wireless device, which, as described above, is a disadvantage for users and manufacturers of these battery operated wireless devices.